Prevalence and risk factors for severe linezolid-associated thrombocytopenia in pediatric patients: An analysis of a public database

Linezolid is widely used in various clinical settings. Studies have revealed that it may cause thrombocytopenia in adults. However, the correlation between the use of linezolid and thrombocytopenia in pediatric patients is still unclear. This study aimed to identify the impact of Linezolid on the occurrence of thrombocytopenia in children. A retrospective observational study was conducted using data on patients treated with linezolid from the Pediatric Intensive Care clinical database. Univariate and multiple logistic regression analyses were performed to identify the risk factors of linezolid-related severe thrombocytopenia. A total of 134 patients were included. The prevalence of severe thrombocytopenia was 8.96% (12/134). Univariate analysis indicated that the severe thrombocytopenia group showed significantly higher proportion of concomitant carbapenem (75% vs 44.3%; P < .05) and piperacillin/tazobactam (25% vs 6.6%; P < .05) than that of the non-severe thrombocytopenia group. Multivariate analysis also revealed that the occurrence of severe thrombocytopenia was significantly associated with concurrent use of carbapenem (odd ratio = 4.058; 95% confidence interval: 1.012–16.274; P = .048) and piperacillin/tazobactam (odd ratio = 5.335; 95% confidence interval: 1.117–25.478; P = .036). 75% of patients (9/12) developed severe thrombocytopenia within the first 7 days of linezolid use. The concomitant use of carbapenem and piperacillin/tazobactam was associated with an increased probability of severe thrombocytopenia in pediatric patients undergoing linezolid treatment. Further prospective clinical studies are required, and more detailed mechanisms of blood toxicity in pediatric patients must be investigated.


Introduction
Linezolid is the first member of the oxazolidinone family and is widely used in clinical settings because of its potent antimicrobial activity against methicillin-resistant Staphylococcus aureus, penicillin-resistant Streptococcus pneumoniae, vancomycin-resistant Enterococcus, and other multidrug-resistant gram-positive bacteria. [1] Linezolid exerts its antibacterial activity by binding to the 23S site of ribosomal RNA on the 50S subunit of bacteria, inhibiting 70S complex formation and bacterial protein synthesis. [2] The absorption of linezolid is both rapid and complete after oral administration, resulting in a 100% absolute bioavailability rate. Thus, there is no requirement to adjust the dosage of Linezolid when switching between oral and intravenous administration. [3] This characteristic supports SY and WC contributed equally to this work. The data that support the findings of this study are available from a third party, but restrictions apply to the availability of these data, which were used under license for the current study, and so are not publicly available. Data are available from the authors upon reasonable request and with permission of the third party.
Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). the practice of sequential oral administration of linezolid following initial intravenous administration. The plasma protein binding rate of linezolid was found to be approximately 31% and was not concentration-dependent. In healthy volunteers, the distribution volume of linezolid in stable state was 40 to 50L on average, which is close to the total body fluid. Both animal and human studies have demonstrated that linezolid has strong tissue penetration and can be rapidly distributed in well-perfused tissues. [4,5] Linezolid is associated with several common adverse effects including diarrhea, headache, nausea, vomiting, and insomnia. Laboratory tests may reveal a decreased platelet count, decreased White blood cell count, or abnormal liver function. Patients undergoing longer treatment courses may face an increased risk of developing peripheral neuropathy and optic neuropathy. [6] It is important to note that Linezolid is a mildly reversible, nonselective monoamine oxidase inhibitor that can interact with epinephrine or serotonin drugs to produce symptoms of serotonin syndrome, such as fever and elevated blood pressure. [7] Nowadays, thrombocytopenia caused by the drug has attracted increasing attention. A few studies have reported some risk factors in patients treated with linezolid for thrombocytopenia, such as poorer renal function, longer treatment duration, and the coexisting condition of malignancy. [8][9][10] Most studies conducted to date concerning this issue have always focused on adult populations, with only a few demonstrating the impact of linezolid use on the development of thrombocytopenia in pediatric patients. [11,12] Therefore, we conducted this study to investigate the relationship between linezolid use and severe thrombocytopenia and identify the potential risk factors for severe linezolid-associated thrombocytopenia in pediatric patients.

Data collection
This retrospective observational study was conducted according to the Declaration of Helsinki (as revised in 2013) using data from the Pediatric Intensive Care database (PIC database, version 1.1). This is a publicly accessible single-center database containing the clinical information of approximately 13,000 patients admitted to the Children Hospital of Zhejiang University (Hangzhou, Zhejiang, China) from 2010 to 2018. [13] The following data of each patient were extracted using PostgreSQL (Version 9.6) and R language (Version 3.6.0) by 2 independent researchers: demographic characteristics, diagnosis at admission/discharge, dates and results of laboratory tests and prescriptions, and other related data. Baseline laboratory tests were defined as the latest test taken within 72 hours before the start of treatment. Concomitant drugs were defined as medications administered for an overlapping duration of more than 48 hours with linezolid treatment.

Patient selection
Individuals aged between 28 days and 18 years are defined as pediatric patients in our study. All pediatric patients treated with linezolid for 5 days or longer were included. The exclusion criteria were patients without baseline platelet counts, patients with baseline platelet counts of <100 × 10 9 /L, patients diagnosed with hemato-oncologic diseases and bleeding disorders, and patients without monitoring of platelet counts (at least every 48 hours) during linezolid therapy. Severe thrombocytopenia was defined as a reduction in platelet counts of >75% from baseline or <50 × 10 9 /L. The study protocol was approved by the Institutional Review Board of the Children Hospital, Zhejiang University School of Medicine (Hangzhou, China). The requirement for obtaining individual patient consent was waived as the study did not have an impact on clinical care and all protected health information was de-identified.

Statistical analysis
Data were processed with SPSS for Windows (Version 22.0). Continuous variables were analyzed using the Student t test or Mann-Whitney U test, whereas categorical variables were analyzed using the chi-squared test or Fisher exact test. Variables with P value < .1 in univariate analysis were analyzed using logistic regression. Statistical significance was set at P < .05. Missing variables were addressed with the multiple imputation technique.

Patient cohort
Of the patients in the PIC database, 320 received linezolid treatment for 5 days or longer. Among them, 186 patients were excluded based on the exclusion criteria. The remaining 134 patients were included for further analysis (Fig. 1).

Linezolid use and development of severe thrombocytopenia
During linezolid treatment, 12 patients (8.96%) developed severe thrombocytopenia ( Table 2). The risk factors associated with thrombocytopenia were investigated using univariate analysis between 2 groups (Table 3). In the severe thrombocytopenia group, the concomitant use of carbapenem and piperacillin/ tazobactam was significantly higher than that in the non-severe thrombocytopenia group (75% vs 44.3%, P < .05; 25% vs 6.6%, P < .05; respectively). There was no significant difference between the 2 groups in linezolid treatment duration, baseline laboratory data (including hemoglobin counts, platelet counts, and serum creatinine level), and use of other concomitant drugs (including sulfamethoxazole-trimethoprim, valproic acid, and heparin/low molecular weight heparin). Further multivariate analysis also revealed that the concomitant use of carbapenem (odd ratio = 4.058; 95% confidence interval: 1.012-16.274; P = .048) and piperacillin/tazobactam (odd ratio = 5.335; 95% confidence interval: 1.117-25.478; P = .036) with linezolid was a risk factor for the development of severe linezolid-associated thrombocytopenia.

Discussion
To our knowledge, this is the first study to investigate the prevalence and risk factors of linezolid-associated severe thrombocytopenia in pediatric patients in China. Results showed that 8.96% of patients receiving linezolid treatment developed severe thrombocytopenia, and the concomitant use of carbapenem and piperacillin/tazobactam may elevate the risk of this adverse event.
Previous studies have indicated that linezolid-associated hematologic adverse events often occurred at least 14 to 21 days after the start of treatment. [14][15][16] Prolonged treatment duration increases the probability of hematologic adverse events caused by linezolid. A recent meta-analysis showed that the incidence of thrombocytopenia increased by 5 times in pediatric patients receiving linezolid treatment for more than 14 days. [17] In the present study, we could not identify long-term treatment as a risk factor for linezolid-associated severe thrombocytopenia. However, most cases of severe thrombocytopenia occurred within the first 10 days of linezolid treatment. Hence, severe thrombocytopenia may occur earlier than we expected in the course of antimicrobial therapy, and careful dynamic monitoring of platelet counts might be essential.
We also analyzed the use of concomitant drugs, including several antibiotics, antiseizure medications (valproic acid), heparin, and low-molecular-weight heparin that may induce thrombocytopenia. Results showed that the concomitant use of carbapenem and piperacillin/tazobactam with linezolid was a risk factor for the development of severe thrombocytopenia in pediatric patients. This finding was similar to that reported by Chen et al [16] and Kılıç et al [18] who found that adult patients undergoing carbapenem-linezolid combination therapy had higher risk of developing thrombocytopenia. It must be noted that the concomitant use of linezolid and carbapenem or piperacillin/tazobactam provides broad antibacterial spectrum and is widely applied in clinical settings, especially for patients with severe conditions or as empiric treatment. Therefore, it is necessary to focus more on correlating the adverse event of thrombocytopenia of this regimen and the potential mechanism. Interestingly, carbapenem and piperacillin/tazobactam, both of which are from the group of β-lactam antimicrobials, are rarely considered as the cause of thrombocytopenia. There is limited literature on thrombocytopenia induced by carbapenem or piperacillin/tazobactam. [19][20][21] The mechanism by which the combination therapy increases the risk is unclear. The possible explanations include the additional myelosuppression effect that inhibits the production of platelets and the accelerated activation of the complement system to promote the destruction of platelets. [22][23][24] However, further studies are necessary. We also analyzed other potential risk factors associated with linezolid-induced thrombocytopenia, including renal function and baseline platelet counts, but we found no correlation. However, Hanai et al [14] reported increased risk of developing linezolid-associated thrombocytopenia in adult patients when their creatinine clearance was <60 mL/min. In pediatric patients, Johns et al [11] found that renal impairment before the start of linezolid treatment was an independent risk factor for thrombocytopenia. Although linezolid is eliminated from the body primarily through nonrenal pathways, elevated drug concentrations can be found in patients with renal insufficiency, increasing the probability of thrombocytopenia. Our failure to identify the correlation between renal function and severe thrombocytopenia may be explained by the fact that we could use only baseline serum creatinine as the index of patients' renal function, instead of estimated glomerular filtration rate or creatinine clearance, due to missing data in the database, such as height and weight. We also found no correlation between drug dosage and decline in platelet count, since the dose of linezolid  must be adjusted according to the weight of patient in populations younger than 12 years old, the absence of complete data may lead to a bias in the results. A few studies have revealed the relationship between baseline platelet counts and thrombocytopenia development. For instance, Choi et al [25] reported that a baseline platelet counts of <150 × 10 9 /L was associated with thrombocytopenia occurrence, whereas Lima et al identified a baseline platelet counts of <200 × 10 9 /L. [26] Our failure to detect a significant correlation may be attributed to the relatively small simple size.
Compared to vancomycin, linezolid has the advantage of oral administration and a higher bioavailability, making it a commonly utilized option in sequential treatment regimens for long-term management of infectious diseases. In clinical practice, there have been instances where patients continue to take linezolid orally after they leave the hospital. This presents a significant challenge and emphasizes the need for adequate patient education regarding potential adverse drug reactions and the conditions that may augment their likelihood. Proactive monitoring of patient outcomes through methods such as telephonic follow-up or establishment of a support group is imperative to detect any adverse events in a timely manner. In case of severe adverse reactions, such as severe thrombocytopenia, prompt notification of the patient family and initiation of appropriate medical intervention is crucial.
There were inherent limitations in this study. First, as mentioned earlier, the inaccessibility of certain information in the PIC database, such as height, weight, and serum drug concentration, may negatively affect our conclusion. Second, the retrospective nature of this single-center study and the small sample size may restrict the generalization of our result.

Conclusion
The concomitant use of carbapenem and piperacillin/tazobactam was associated with an increased probability of severe thrombocytopenia in pediatric patients undergoing linezolid treatment. However, due to the small sample size and missing data of clinical information, further prospective clinical studies are required, and more detailed mechanisms of blood toxicity in pediatric patients must be investigated.